A conventional yoke-trunnion universal joint has a complicated structure between its trunnion ends and sleeves, leaving a large clearance there between. This, in turn, leads to loose construction and reduced strength. Moreover, needle bearings are rather complicated to manufacture, process and assemble and they easily wear out and run with a loud noise. For example, one kind of universal joint has been applied for patent (Chinese application No.: 87013541) that needs filling with lubricating oil to assure uniform and long-term lubrication at all four roller bearings. To meet these needs, two springs are installed in two vertical passages of the yoke-trunnion. Before installation, the length of the springs is greater than the axial size of the passage. After installation, both ends of each spring contact the inner end surface of the corresponding bearing housing. Such universal joint is quite complicated and the springs lead to a loose structure and high assembly difficulty, tend to wear out, run with loud noise and are hard to seal tightly. Oil leakage often occurs so that the universal joint has a shortened service life and is not economical.
U.S. Pat. No. 6,881,151 teaches a universal joint without needle bearings. Its center cub and rod ends are molded as one integral plastic unit, which make the universal joint weak, can not satisfy the requirements for transmission. Therefore, to date it is not popular in the market.
U.S. Pat. No. 6,881,151 teaches a yoke-trunnion universal joint without needle bearings. There is an anti-wear coating between the inner wall surface of a cup and the trunnion. The wear resistant materials include a material selected from the group consisting of chromium nitride, titanium nitride, titanium carbonitride, zirconium nitride, aluminum titanium nitride, titanium aluminum, zirconium nitride, and aluminum oxide. As the anti-wear coating is consisted of metallic compound, it still needs lubrication. Therefore, the yoke-trunnion universal joint without needle bearings still needs complex lubricant system. Further more, the process of making the metallic compound coating is quite complex. The anti-wear coating is easy to separate from the body. The cost for manufacture is high.